Silicon and ferrosilicon are obtained by subjecting quartz, when silicon is desired, or quartz and an iron bearing material, when ferrosilicon is desired, to a reduction with coke or coal and woodchips in an open electric arc furnace. The quartz and coke or coal used must be as pure as possible in order to obtain the purest silicon or ferrosilicon possible.
Quartz is one of the crystalline form of silicon dioxide and its reduction with carbon does not follow the reaction: ##STR1##
In fact, there is an intermediate reaction which takes place wherein a suboxide of silicon is formed according to the reaction: ##STR2##
Part of the gaseous suboxide of silicon which is formed escapes with the CO gas and will then react with the oxygen in the air in accordance with the reaction: ##STR3##
The gases which escape from the furnace will contain minor amounts of other metallic gases which are mainly very volatile. Alkali metals, such as potassium and sodium, condense at the same time as the suboxide of silicon to form glassy microscopic particles. These glassy microscopic particles are known as silica fumes, volatilized silica or submicron silica. These particles are collected through various filtering systems. The amount of silica fumes or volatilized silica represents about 250 to 500 kilograms per ton of metal produced. The expression `volatilized silica`, whenever used herein, is intended to cover the very fine silica particles which are usually referred to either as volatilized silica, silica fumes or silica dust and also volatilized ferrosilicon, ferrosilicon fumes or ferrosilicon dust.
Further, volatilized silica is characterized by having a very fine particle size. The average diameter of the glassy microscopic particles varies from 0.5 to 1.0 .mu.m, this diameter being from 50 to 100 times smaller than that of Portland cement or fly ash. Also, because of its microscopic size, the specific surface of volatilized silica can be as high as 20,000 m.sup.2 /kg, which is at least 50 times greater than that of Portland cement or fly ash.
The chemical composition of volatilized silica will vary slightly whether it is obtained from the manufacture of silicon or ferrosilicon, and generally the silicon dioxide content will vary from 85% to 95%, the carbon content will be from 2 to 5% while the content of aluminum oxide and calcium oxide will generally be less than 2%.
Furthermore, the chemical composition of volatilized silica is quite different than that of pozzolan and fly ash. The volatilized silica has a SiO.sub.2 content much higher than that of pozzolan and fly ash while the aluminum oxide content of the latter two compounds is substantially greater than that of the volatilized silica dust.
A further distinction between volatilized silica, natural pozzolan and fly ash is that the ASTM standard C 618-78 requires that when substituting 30% of Portland cement with fly ash or pozzolan in a standard mortar, the amount of water to be added must not be increased by more than 15%. When attempting to substitute 30% of Portland cement with volatilized silica, it has been found that 50% water must be added to get a mortar of normal consistency.
Furthermore, it is known that the incorporation of fly ash or pozzolan to concrete requires a few months before an increase in compression strength is noted while in less than one month, a slight decrease of its resistance is observed. When silica fumes are used in concrete, high increases are observed after one week.
The disposal of such large volumes of volatilized silica represents a problem because of the physicochemical properties of volatilized silica. Volatilized silica is very light in weight since its bulk density is from 200 to 250 kg/m.sup.3, which means that 1 kg of volatilized silica occupies a volume which is 4 to 5 times greater than 1 kg of Portland cement or fly ash, two other powders more or less similarly used in concrete.
It will be appreciated that the disposal of large quantities of volatilized silica causes an environmental problem. Accordingly, for most producers of silicon or ferrosilicon, the large volumes of the volatilized silica produced, creates a disposal problem and so far most companies are content to bury the volatilized silica by-product. Nevertheless, this is not an answer to the problem because now environmentalists are taking note of this type of disposal and in certain countries recent legislation has been set up to control or prevent this type of disposal.
It is known that a small amount of volatilized silica could be used in certain countries to replace a small amount of Portland cement in the manufacture of concrete. In this case generally the volatilized silica is sold in the form of a dilute slurry. This operation can be done when the silicon manufacturing plant is within a short distance of the concrete mixing plant and where the climatic condition of the country is such that transporting an aqueous slurry of volatilized silica will not cause any problem in the winter months. This mode of delivery has been chosen because of its lightness and particle size, volatilized silica cannot be transported over any reasonable distance in any kind of tightly closed container nor can it be economically packaged. It is well known that every time the volatilized silica is manipulated in a dry form it creates uncontrollable dust problems so that the user must have a sophisticated and very expensive dust collector system.
Accordingly, it would appear highly desirable if a procedure for disposing of volatilized silica could be developed which would offer various possibilities depending upon the seasonal needs of various types of industries.